The present invention relates to a metal gasket, what is called a cylinder head gasket for use in sealing the joint surfaces of the cylinder head and the cylinder block of an internal combustion engine.
A conventional cylinder head gasket is constructed by laminating a plural number of metal plates each having combustion chamber hole(s), bolt holes, hole(s) for water passage, hole(s) for oil passage and the like: for example, an upper plate 101, intermediate plate 104, thickness-adjusting plate 103 having a thickness smaller than that of the upper plate 101, and lower plate 102 are laminated as shown in FIG. 1, and a bead portion 107 for surrounding the combustion chamber hole 105 is formed on the intermediate plate 104 at a position corresponding to the bent portion 106 of the upper plate 101. The bead portion, as shown in FIG. 2, in general, has a projection 203 formed on one surface 201 of the metal plate 200 and a corresponding recess 204, which is formed on the other side of the projection 203, in the other surface 202 thereof. The term "bead portion" referred to in the present specification means such a bead portion as mentioned above.
In the above cylinder head gasket, the thickness of a part of the gasket in which the bent portion of the peripheral portion of the combustion chamber hole 105 is disposed is larger than that of the remaining part of the gasket. In this regard, the wording "the thickness of a part of the gasket" referred to in the present specification means the sum of the thickness of the metal plates and the thickness of a rigid resin layer which are laminated at the relevant part. When such a gasket is fastened between the joint surfaces of a cylinder head and a cylinder block, an annular high sealing pressure zone takes place around the peripheral portion of the combustion chamber hole 105 due to the above difference in partial thickness of the gasket. The high sealing pressure is further increased by addition of a sealing pressure resulting from the resilience of the bead portion 107.
It is, however, difficult to construct a gasket with a difference in partial thickness between the peripheral portion of the combustion chamber hole 105 and its outside portion of the gasket so that a desired sealing pressure distribution can be created, because the above difference in partial thickness is determined by the difference in thickness between the metal plates 101 and 103. Further, the fastening force the gasket receives when fastened differs depending on a distance from a bolt hole, and therefore, the sealing pressure applied to the peripheral portion of the combustion chamber hole differs depending on a distance from the bolt hole. To uniform the sealing pressure, it is required to vary and adjust the above difference in partial thickness of the gasket in accordance with a distance from the bolt hole. Such adjustment is however practically impossible for the laminate gasket as mentioned above.
The assignee of the present invention has previously proposed the following metal gasket in Japanese Patent Application No. 8-38787 (Japanese Laid-Open Patent Publication No. 9-210205 issued on Aug. 12, 1997): the metal gasket, as shown in FIG. 3, comprises a resilient metal plate 100 and a bead portion 108 for surrounding a combustion chamber hole 105, wherein a rigid resin layer 110 is disposed in the recess 109 of the bead portion 108 and the thickness of the rigid resin layer 110 is varied in accordance with a distance from a bolt hole (not shown).
The rigid resin layer 110 of the invention of the above application serves as a stopper for limiting the compression degree of the bead portion 108. The provision of the rigid resin layer 110 thickens the part of the gasket in which the rigid resin layer 110 is disposed more than the remaining part of the gasket, thereby causing a higher sealing pressure around the combustion chamber hole 105. Further, the thickness of the rigid resin layer 110 can be adjusted freely, and adjustment of the thickness thereof in accordance with a distance from the bolt hole can be easily made.
The gasket shown in FIG. 3, however, has the following problems. When the gasket is applied between a cylinder head and a cylinder block, the rigid resin layer comes into contact with the joint surfaces of the cylinder head and the cylinder block, and therefore, the rigid resin layer is worn down by friction with the joint surfaces due to vibrations of the cylinder head and the cylinder block during operation of the engine and due to the expansion and contraction of the cylinder head and the cylinder block accompanying the start and stop of operation of the engine. Further, there is another problem that the rigid resin layer deteriorates due to exposure to a high temperature combustion gas.